Applying voltage drop to a motor calculation, wanting to

Status
Not open for further replies.
Y

yanert

Member
Applying voltage drop to a motor calculation, wanting to find wire size.

VD formula, single phase, finding Circular Mils.

CM=2xKxIxD divided by total voltage drop.

When I use this formula, what number would I put in for amperage? The FLC of the motor nameplate, or would I use the motor tables in the code?

Also how would I apply the 125% to the conductors for sizing the wire.

I have always been able to find the voltage drop for a ckt, but I have never applied it to a motor. I can?t seem to find anything in the code to give me some direction.

Single phase, 5hp motor, 115E, 52I FLC. 110 feet away. Copper
K = 12.9, constant for copper wire
I = 52 FLA motor nameplate, or 56I, table 430.148
D= 110 feet
VD=115Ex.03% drop max. 3.45
Using both load current numbers, the wire is a #3 AWG, but what load would I use, nameplate or table, and where is it in the code, if at all.
Also, it would appear that I found the circ mils for a conductor at 100% of the full load, (52 I nameplate, 56I table) regardless of what figure I use.
430.22 tells me to take 125% of the full load current to size the conductor. So I am just a bit confused. Just hoping somebody can help me get my mind wrapped around this one.
 
Re: Applying voltage drop to a motor calculation, wanting to

What I would do is first determine the NEC required branch circuit conductor size by taking the FLC per the table and increasing that value by 125%. I would then perform a VD formula to determine how much voltage drop I would have using the NEC required minimum. If that conductor size raises the voltage drop above the % desired for the application, I would then perform a CM formula to determine the needed size to meet that % of drop using the actual nameplate runnng amperes of the motor. There are no rules for voltage drop calculations.

On a side note. 12.9 K and 3% VD is fairly conservative for typical circuits. Most equipment has greater tolerances.
 
Re: Applying voltage drop to a motor calculation, wanting to

Most of the time you don't have to look too hard at the voltage drop on a motor circuit because the conductors must be sized at 125% of the motor current from the code tables. In this case the minimum conductor size is #4 copper. The online calculator that I use shows a 3.2 volt drop using
#4 at a 52 amp load and 110'. That is 2.7% at 120 volts.
Don
 
Re: Applying voltage drop to a motor calculation, wanting to

Maybe I'm just unlucky, but I often run into very long motor circuits. We have a MCC room and motors scattered for hundreds of feet. Voltage drop does become a problem.

I would suggest two views.
1. If you are designing something, use the Table flc's. This is close to a worst case scenario.
2. If you are troubleshooting a problem with a specific motor, use nameplate flc, or better yet read the actual current with an ammeter.

I don't believe there is any specific direction on this matter in the Code.

Jim T
 
Re: Applying voltage drop to a motor calculation, wanting to

Jim,
I have the same type of installation with central motor control centers and long runs to the 480 volt motors. Many times the code required conductor size is less than 3% drop up to about 500-600'. I just installed a 100hp motor 600' from the MCC. Table 430.150 load for this motor is 124 amps and the minimum code permitted conductor is 155 amps. This requires the use of 2/0 copper. My calculator shows a 2.4% drop for this installation using the motor load of 124 amps. It also shows that I could go 750' before I exceeded a 3% drop.
Don
 
Re: Applying voltage drop to a motor calculation, wanting to

Don
I think it's a case by case problem. I calculated a 10hp with #12 wire, a 20 hp with #10 wire and a 50 hp with #4 wire, all at 600 ft, and got the following ,
10 hp 5.15%
20 hp 6.4%
50 hp 4.2%
Jim T
 
Re: Applying voltage drop to a motor calculation, wanting to

Jim,
Yes, it does make a difference what horsepower the motor is.
Don
 
Re: Applying voltage drop to a motor calculation, wanting to

The NEC is not a substitute for a good engineering. When sizing a very long motor cable a VD caused by the inrush current should be taken into consideration.

In addition, 3% recommended VD assume that the supply voltage is constant. In NYC during the summer months Con Edison regularly reduces the voltage by 5% to reduce the demand. Since the FLC rises accordingly due to the lower voltage VD becomes a big problem.

Sometime the VD is very useful. Years a go I used the VD to provide reduced voltage start for a 1500 HP (bow thruster motor for a tanker)

So it is case by case and make sure that 125% makes it legal.
 
Re: Applying voltage drop to a motor calculation, wanting to

Originally posted by oliver100:
In NYC during the summer months Con Edison regularly reduces the voltage by 5% to reduce the demand. Since the FLC rises accordingly due to the lower voltage VD becomes a big problem.
Click to expand...
Only if the mechanical output of the motor remains constant. ;)

I do like your bow thruster 'soft start'. :cool:

I was going to mention the reduced voltage at start up may be good thing, then I got to the end of your post and you covered that anyway. :cool:
 
Status
Not open for further replies.
Top